With increasing demand in recent years for higher performance and longer operating time of various devices such as cellular phones, portable information devices, laptop computers, camcorders, and portable game players, electrochemical energy storage devices incorporated in those devices are required to have a higher energy density.
Lithium ion batteries, one of electrochemical energy storage devices, include as a positive electrode active material, for example, lithium cobalt oxide (LiCoO2). When charged, lithium cobalt oxide releases lithium ion and converts into, for example, Li0.5CoO2. When discharged, the lithium cobalt oxide absorbs lithium ion and converts into LiCoO2 again. Note that Li0.5CoO2 in a charged state has an electric capacity of as small as 142 mAh/g. The discharge reaction is represented by the reaction formula (1):4Li0.5CoO2+2Li++2e→4LiCoO2  (1).
A negative electrode active material used for lithium ion batteries is, for example, an intercalation compound of lithium and graphite. Even in the composition of C6Li, the electric capacity thereof is 339 mAh/g.
As shown above, as long as conventional active materials are used, further improvement of energy density of lithium ion batteries cannot be expected. Therefore, use of an active material having an electric capacity higher than that of lithium cobalt oxide or intercalation compound (C6Li) has been required.
For example, use of FeF2 as a positive electrode active material has been examined. When discharged, FeF2 reacts with lithium and decomposes into iron metal (Fe) and lithium fluoride (LiF). This reaction is represented by the reaction formula (2):FeF2+2Li++2e→Fe+2LiF  (2).
Comparison between the reaction formulas (1) and (2) shows that the formula weight of FeF2 is much smaller than that of 4Li0.5CoO2. Therefore, the use of FeF2 as a positive electrode active material can significantly increase the electric capacity. Such reaction is known as a conversion reaction. FeF2 has a theoretical electric capacity of 571 mAh/g (see Non Patent Literature 1).
However, in a conversion reaction, a great difference occurs between the electric potentials at the electrode for oxidation (charge at the positive electrode, discharge at the negative electrode) and for reduction (discharge at the positive electrode, charge at the negative electrode); in other words, hysteresis exists. For example, in the case of FeF2, hysteresis of 1 V or more exists between charge and discharge at 60° C. (see Non Patent Literature 2). This means not only that the energy inputted during charge is wasted, but also that the discharge voltage is reduced more than expected. In short, theoretically, a high electric capacity can be expected, but due to the existence of hysteresis of as high as 1 V, a power storage device having a high energy density is difficult to achieve in the end.
Furthermore, a conversion reaction as represented by the reaction formula (2) does not always occur easily. In order to render FeF2 electrochemically active, it is necessary to pulverize FeF2 into fine particles of nanometer size. Moreover, in order to use FeF2 as an active material for a battery, it is necessary to bring an electrically conductive material such as carbon material into contact with the surfaces of the fine particles.
As for the negative electrode active material, use of an alkaline earth metal, instead of the intercalation compound (C6Li), has been examined. For example, use of magnesium metal (3830 mAh/cm3) and use of calcium metal (2070 mAh/cm3) have been examined.
In order to use magnesium metal as a negative electrode active material, it is necessary to use a non-aqueous electrolyte having magnesium ion conductivity. However, no such electrolyte as satisfying the required properties has been obtained. Although there is reported an electrolyte that can electrochemically precipitate and dissolve magnesium metal, such an electrolyte has problems in the stability etc. When importance is placed on the stability of the electrolyte, however, magnesium metal cannot be properly precipitated and dissolved in the electrolyte.
For example, there is reported a non-aqueous electrolyte in which magnesium chloride is dissolved in tetrahydrofuran (THF) (see Patent Literature 1). It is necessary, however, to add dimethylaluminum chloride ((CH3)2AlCl) to the non-aqueous electrolyte. This electrolyte, in which a complex having a plurality of magnesium ions as nucleus and a complex having aluminum ion are considered to be produced, can electrochemically precipitate and dissolve magnesium metal. Disadvantageously, however, dimethylaluminum chloride is highly combustible and highly corrosive, and therefore, is difficult to handle.
Another report says that an electrolyte with magnesium ion conductivity can be obtained by heating magnesium metal at 60° C. in, for example, 1,2-dimethoxyethane, methyl trifluoromethanesulfonate, tetrabutylammonium tetrafluoroborate, or aluminum chloride. The report says that discharge reaction is possible in this electrolyte, when manganese oxide is used as a positive electrode, and magnesium metal is used as a negative electrode (see Patent Literature 2). Through this reaction, magnesium metal is electrochemically dissolved as shown in the reaction formula (3):Mg→Mg2+2e  (3).
However, methyl trifluoromethanesulfonate is an essential component of a non-aqueous electrolyte with magnesium ion conductivity, and in a non-aqueous electrolyte not containing this component, discharge is impossible. Moreover, no report says that charge reaction represented by the reaction formula (4) is possible:Mg2+2e→Mg  (4).
Under these circumstances, the present inventors have studied and found that magnesium metal is difficult to produce according to the reaction formula (4), in the electrolyte proposed by Patent Literature 2. They also found that methyl trifluoromethanesulfonate, an essential component of the electrolyte, reacts with moisture as impurities, to produce trifluoromethane sulfonic acid. The trifluoromethane sulfonic acid causes the positive electrode active material, current collector, and metal case, to corrode, and the corrosion becomes severe as the electric potential at the positive electrode increases.